The global positioning system is a satellite-based navigation system consisting of a network of up to 32 orbiting satellites (called space vehicles, “SV”) that are in six different orbital planes. 24 satellites are required by the system design, but more satellites provide improved coverage. The satellites are constantly moving, making two complete orbits around the Earth in just under 24 hours.
Each satellite transmits on several frequencies. Civilian GPS receivers currently use the ‘L1’ frequency of 1575.42 MHz. The satellite orbits are roughly 25,000 kilometers from the earth's centre, or 20,000 kms above the earth's surface. The orbital paths of these satellites take them between roughly 60 degrees North and 60 degrees South latitudes.
The GPS signals transmitted by the satellites are of a form commonly known as Direct Sequence Spread Spectrum employing a pseudo-random code which is repeated continuously in a regular manner. The satellites broadcast several signals with different spreading codes including the Coarse/Acquisition or C/A code, which is freely available to the public, and the restricted Precise code, or P-code, usually reserved for military applications. The C/A code is a 1,023 bit long pseudo-random code broadcast with a chipping rate of 1.023 MHz, repeating every millisecond. Each satellite sends a distinct C/A code, which allows it to be uniquely identified.
A data message is modulated on top of the C/A code by each satellite and contains important information such as detailed orbital parameters of the transmitting satellite (called ephemeris), information on errors in the satellite's clock, status of the satellite (healthy or unhealthy), current date, and time. This part of the signal is essential to a GPS receiver determining an accurate position. Each satellite only transmits ephemeris and detailed clock correction parameters for itself and therefore an unaided GPS receiver must process the appropriate parts of the data message of each satellite it wants to use in a position calculation.
The data message also contains the so called almanac, which comprises less accurate information about all the other satellites and is updated less frequently. The almanac data allows a GPS receiver to estimate where each GPS satellite should be at any time throughout the day so that the receiver can choose which satellites to search for more efficiently. Each satellite transmits almanac data showing the orbital information for every satellite in the system.
A conventional GPS receiver reads the transmitted data message and saves the ephemeris, almanac and other data for continual use. This information can also be used to set (or correct) the clock within the GPS receiver.
To determine position, the GPS receiver compares the time a signal was transmitted by a satellite with the time it was received by the GPS receiver. The time difference tells the GPS receiver how far away that particular satellite is. By combining distance measurements from multiple satellites, position can be obtained by trilateration. With a minimum of three satellites, a GPS receiver can determine a latitude/longitude position (a 2D position fix). With four or more satellites, a GPS receiver can determine a 3D position which includes latitude, longitude, and altitude.
By processing the apparent Doppler shifts of the signals from the satellites, a GPS receiver can also accurately provide speed and direction of travel (referred to as ‘ground speed’ and ‘ground track’).
A complete data signal from the satellites consists of a 37,500 bit Navigation Message, which takes 12.5 minutes to send at 50 bps. The data signal is divided into 25 30 s frames, each having 1500 bits, and these are divided into five 6 s subframes. Each 6 s subframe is divided into ten 30 bit words. All the information necessary for a position fix (ephemeris etc) is contained within each frame and so a GPS receiver will typically take around 30 s to produce a position fix from a so-called cold start.
The first word of each 6 s subframe contains a preamble to enable receivers to recognise the start of the subframe. The next word is the handover word which gives timing information since the last restart of GPS time (which takes place every Saturday/Sunday at midnight). The remaining 8 words provide the ephemeris, almanac and clock information, depending on the subframe.
The first subframe gives clock correction data, the second and third subframes give ephemeris data and the almanac data is in the fourth and fifth subframes.
The SVs all broadcast on the same frequency. In order to distinguish a signal from a particular satellite, the receiver needs to generate a replica of the C/A code known to be in use by that satellite and align it so that it is synchronised with the incoming signal which will be delayed by an unknown amount predominantly due to the time of flight of the signal in travelling from the satellite to the receiver (typically around 0.07 s). In general it is not possible for a receiver to accurately predict the alignment necessary to get the replica in sync with the incoming signal, so some form of search is required, with a number of alignments being tried in turn and the best match being selected. This process of evaluating a number of candidate alignments is normally termed correlation as the receiver implements a correlation function between the received signal and the known C/A code for each satellite in turn, to determine if the received signal includes a component having the C/A code from a particular SV. The correlation function has to be calculated for multiple relative timings, and when the correlation peak is found, this corresponds to a particular timing and a particular SV. The discovered timing in turn corresponds to a particular distance from the SV.
The determined code phase, i.e. the timing of the peak of the correlation function, reveals the accurate timing information for use in the distance calculation. However, as the code is repeated every millisecond, the coarse timing also needs to be determined. Typically, less frequently repeating data components are used for the more coarse timing evaluation (i.e. to enable GPS time to be derived), such as the individual bits of the 50 bps data message and specific parts of it such as the subframe preamble or subframe handover word.
The integration period over which the correlation function is performed determines how easily a signal encoded with a particular code can be identified; in general, the longer the integration period, the more reliably the correct code phase should be reported for a given signal strength. The weaker the signal to be detected, the longer the integration period needs to be to achieve the same level of reliability.
Nearly all current and historical GPS receivers work by processing signals from the satellites in “real time”, as they come in, reporting the position of the device at the current time. Techniques associated with such conventional receivers are well known.
Such “conventional” GPS receivers invariably comprise:                an antenna suitable for receiving the GPS signals,        analogue RF circuitry (often called a GPS front end) designed to amplify, filter, and mix down to an intermediate frequency (IF) the desired signals so they can be passed through an appropriate analogue-to-digital (A/D) convertor at a sample rate normally of the order of a few MHz,        digital signal processing hardware that carries out the correlation process on the IF data samples generated by the A/D converter, normally combined with some form of micro controller that carries out the “higher level” processing necessary to control the signal processing hardware and calculate the desired position fixes        
However, the less well known concept of “Store and Process Later” has also been investigated. This involves storing the IF data samples collected by a conventional antenna and analogue RF circuitry in some form of memory before processing them at some later time (seconds, minutes, hours or even days) and often at some other location, where processing resources are greater.
The key advantages of the Store and Process Later approach over conventional GPS receivers are that the cost and power consumption of the capturing device are kept to a minimum as no digital signal processing needs be done at the time of capture, and the grabs can be very short (e.g. 100 ms). If the subsequent signal processing is done when the relevant satellite data (ephemeris etc) can be obtained via some other method, this approach also removes the need to decode the (very slow) data message from the SVs in the capturing device, which in many cases leads to unacceptably long times to start up conventional devices.
For example, a number of small capture devices which store short “grabs” of IF data samples into memory can subsequently upload their IF data grabs to a shared central computer which would not only carry out the necessary signal processing (correlation etc), but would also have access to a database of recent satellite information (ephemeris etc) by being connected to one or more conventional GPS receivers which relayed key parts of the GPS data message they received to the central computer.
However, the ease and effectiveness of subsequent processing is affected by the manner in which the IF data is collected. It is desirable to store as little data as possible to reduce memory and processing requirements, but this makes obtaining a position fix more difficult.